1. Field of the Invention
The invention relates to a clutch device, particularly to a stationary cylinder type clutch device where a hydraulic servo for operating a clutch device is installed at a fixed member by separating the hydraulic servo from a clutch drum integrated with a friction coupling element.
2. Description of Related Art
Conventionally, according to a multi-plate clutch for connecting a rotational member on the input side with a rotational member on the output side in a speed changing mechanism incorporated in a gear train of an automatic transmission for transmitting and cutting power, either ones of disks and plates of friction coupling elements alternately laminated in the axial direction among a plurality of friction disks by interposing separator plates, are supported unpivotably by an outer peripheral face of a hub by means of a spline coupling and the other ones are similarly unpivotably supported by an inner peripheral face of a drum by means of the spline coupling. According to a hydraulic servo for pressing such friction coupling elements, the hydraulic servo is installed to the clutch drum side fixed to one of the rotation members where a hydraulic chamber is demarcated by a cylinder integrated with a drum and a piston slidably arranged therein and where the piston is extracted by a hydraulic pressure supplied to the hydraulic chamber by which the friction coupling elements are pressed to the drum against a reaction force thereby engaging the clutch. That is, the hydraulic servo is of a rotational cylinder type rotating along with the clutch drum.
According to such a rotational cylinder type clutch, the control of the hydraulic servo becomes complicated since a centrifugal hydraulic pressure is generated by rotating the cylinder. Further, the hydraulic pressure must be supplied from a fixed member to the rotating cylinder for operating the hydraulic servo. Accordingly, the provision of sealing for preventing oil leakage from a relatively rotating portion in an oil supply passage is indispensable whereby an abrasive resistance caused by dragging the seal ring is caused and deterioration of a transmission efficiency by the amount of resistance is inevitable. Hence, there has been proposed a stationary cylinder type clutch capable of resolving the problem accompanied by the rotation of the hydraulic servo by separating the hydraulic servo from the clutch drum and installing it on the side of a fixed member, such as a casing. A technology is disclosed, as an example of such a clutch, in Japanese Laid-open Patent Publication No. JP-A-7-119761.
According to the prior art, friction coupling elements are connected to inner and outer peripheries of a hub and a drum structuring input and output members of a clutch, such that the rotational power can be transmitted, a pressing member and a reaction force member are arranged at both ends of the friction coupling elements in the axial direction to interpose the friction coupling elements, a hydraulic servo for engaging and disengaging the friction coupling elements is installed at a fixed member, a servo force of the hydraulic servo is transmitted to the friction coupling elements via the pressing member and a reaction force caused by the engagement of the friction coupling elements is supported by the fixed member via the reaction force member. Further, a return spring, for pushing back a piston of the hydraulic servo for releasing the clutch, is arranged between a casing of a speed changing mechanism forming a cylinder of the hydraulic servo and the piston by following an arrangement similar to that where a return spring is normally arranged between a cylinder and a piston of a hydraulic servo in a rotational cylinder type clutch.
In the meantime, according to the structure where the return spring of the hydraulic servo is arranged between the casing of a speed changing mechanism and the piston as in the above-described prior art, in releasing the clutch, the piston is forcibly retracted to a rearmost end portion of the cylinder by the pushing force of the return spring. In this case, loss of the stroke amount of the piston in moving the piston in a clutch engagement direction, that is, an amount of the stroke whereby the piston reaches a substantial engagement start position of the clutch nullifying clearances among the friction coupling elements by filling the cylinder with oil, is an amount of indispensable clearances for substantially eliminating dragging of the friction coupling elements in releasing the clutch, added with an amount of play that is set in consideration of thermal expansion and the like among the piston, the pressing member, the friction coupling elements, the reaction member and the casing. Thereby, a dispersion among individual products with respect to loss of the stroke amount is increased, since the number of interposed members is more than those of a rotational cylinder type clutch. Accordingly, when the piston is retracted to the rearmost end position as in the prior art, even if the control of the hydraulic pressure for engagement of the clutch is carried out finely and with small error among individual products, the engagement characteristics of the respective products differ by the dispersion in the loss of the stroke amount and, as a result, the controllability deteriorates. Furthermore, a large amount of the piston stroke is always required by retracting the piston to the rearmost position whereby the period of time required for the engagement is prolonged.